Two-stroke gasoline engines have been used as a power source of portable power working machines such as hedge trimmers, brush cutters, chain saws or the like. In a two-stroke engine of this type, a combustion chamber is scavenged by a flow of air-fuel mixture pre-compressed in a crank chamber. More specifically, as the piston ascends, the air-fuel mixture is introduced into the crank chamber, and pre-compressed by the descending piston. Then, during the scavenging stroke, the pre-compressed air-fuel mixture is introduced into the combustion chamber to force waste combustion gas (exhaust gas) out of the combustion chamber and replace it.
As such, the two-stroke engines are configured to scavenge the combustion chamber by using flows of air-fuel mixture, and therefore involve the problem of “blow-by”. That is, a part of the air-fuel mixture, introduced into the combustion chamber but having not burnt, is discharged away from the combustion chamber together with the combustion gas. This “blow-by” phenomenon makes it difficult to take effective measures for emissions cut of two-stroke engines.
To control the “air-fuel mixture blow-by” phenomenon, the “stratified scavenging” technique has been proposed in Document 1 (U.S. Pat. No. 6,571,756), Document 2 (Japanese Laid-open Publication No. H05-33657) and Document 3 (Japanese Laid-open Publication No. 2000-240457). Document 1 proposes to introduce fuel-free air (air not containing a fuel) from a first pair of scavenging ports nearer to an exhaust port and an air-fuel mixture from a second pair of scavenging ports remoter from the exhaust port into a combustion chamber during a scavenging stroke, thereby forming a layer of fuel-free air between the air-fuel mixture and the combustion gas in the combustion chamber.
More particularly, Document 1 proposes to provide the first and second scavenging ports in each of left and right cylinder walls at opposite sides of the exhaust port. The first pair of scavenging ports nearer to the exhaust port and the second pair of scavenging ports remoter from the exhaust port are opened simultaneously, and introduce fuel-free air from the first pair of scavenging ports into the combustion chamber and the air-fuel mixture from the second pair of scavenging ports into the same combustion chamber.
Similarly, Document 2 proposes to provide the first and second scavenging ports in each of left and right cylinder walls at opposite sides of the exhaust port. Thus, the engine first introduces fuel-free air from the first pair of scavenging ports nearer to the exhaust port into the combustion chamber, and next introduces an air-fuel mixture from the second scavenging ports remoter from the exhaust port into the same combustion chamber.
Document 3 proposes to provide a first scavenging port in each of left and right cylinder walls at opposite sides of an exhaust port and a second scavenging port in a location opposed to the exhaust port. In a scavenging stroke, this engine first introduces fuel-free air from the pair of first scavenging ports into a combustion chamber, and next introduces an air-fuel mixture from the pair of second scavenging port opposed to the exhaust port into the same combustion chamber.
Document 4 (Japanese Laid-open Publication No. 2002-129963) also proposes a technique for minimizing the “blow-by of air-fuel mixture” phenomenon. This document proposes to provide first and second scavenging ports in each of left and right cylinder walls at opposite sides of the exhaust port. In a scavenging stroke, fuel-free air is first introduced from the first and second scavenging ports into a combustion chamber, and an air-fuel mixture is next introduced from the first and second scavenging ports into the same combustion chamber.
In the recent society involving discussions on environmental problems, it is an urgent request to further reduce harmful emissions from combustion gases.
It has been acknowledged that there is some limit to the conventional stratified scavenging technique that introduces fuel-free air into the combustion chamber from the first pair of scavenging ports located nearer to the exhaust port while introducing an air-fuel mixture into the same combustion chamber from the second pair of scavenging ports located remoter from the exhaust port as disclosed in the above-discussed Document 2 and others. Under the situation, further improvement is required.